The properties of optoelectronic components, which are based on DFB gratings or DBR gratings, such as for example lasers, laser amplifiers, filters, couplers, detectors, multiplexers, demultiplexers and switches, can be improved with this invention and optimized within certain limits.
The literature discloses a first group of solutions, which treats the production of a variation of the coupling coefficient in the axial direction.
An abrupt change of the coupling coefficient in the longitudinal direction of the component was achieved, for example by means of partial photoresist remasking after a partial dry etching of the DFB grating [for example M. Matsuda et al., Conference on InP and related compounds (1991)].
A continuous axial variation of the coupling coefficient K can be implemented by means of a double exposure of a photoresist layer applied to a wafer surface. After the double exposure, the photoresist contains the information of two superimposed and inter-penetrating homogeneous DFB gratings which, however, differ by .DELTA..LAMBDA. in their grating period. [A. Talneau et al., Electron. Lett. 28, 1395 (1992)]. Depending on the choice of .DELTA..lambda., this process enables in each case only a completely defined function K(z) as well as one and only one component length L, corresponding thereto, if the periodicity of the structure is used with a view to high component yield. The advantages which are offered by an arbitrary change of K(z), cannot be used with this process.
Furthermore, the literature discloses a second group of solutions, which treats the generation of one or more phase shifts over the axial extent of a single or of a small number of grating periods.
Depending on the level of the coupling coefficient, abrupt phase shifts in DFB gratings produce excessive numbers of photons at various strengths at the location of the phase shift or shifts, which can have a negative influence on some properties of optoelectronic components. Abrupt phase shifts are implemented, for example, by means of holographic processes [e.g. T. Numai et al., Jap. J. Appl. Phys. 26, L 1910 (1987)] or by means of electron beam lithography (EL).
The excessive numbers of photons at the location, for example of a .lambda./4 phase shift, can be somewhat weakened by splitting up the total amount of the phase shift over several spatially mutually separated phase shift components (multiple phase-shifts) [e.g. S. Ogita et al. J. Lightwave Technol. 8, 1596 (1990)]. The individual partial phase shifts, however, were also carried out abruptly here by means of electron beam lithography or holographic methods.
Furthermore, the literature discloses a third group of solutions, in which the generation of phase shifts is carried out via the axial variation of the effective refractive index. Thus, phase shifts are implemented, for example, also via a lateral broadening or narrowing of the stripe width of the active zone or of the active zone and adjacent layers over a specific length of the longitudinal component length [e.g. B. H. Soda et al., IEEE J. Quant. Electron. OE-23, 804 (1987), or G. Chen et al. Appl. Phys. Lett. 60, 2586 (1992)]. In this arrangement, the effective refractive index was changed in the longitudinal direction in a quasi-abrupt manner. This leads to undesired additional optical multiple reflections and optical interference effects at those points at which the refractive index changes abruptly. Furthermore, the advantages which are offered by a continuous phase shift distributed arbitrarily over a longer partial section, were not exhausted by a long way.
A further known solution contains a change, linear in the z-direction, of the lateral stripe width of the active layer and adjacent layers over a longitudinal partial section of the component, in order to distribute the phase shift spatially [e.g. Y. Nakano et al., IEEE J. Q. Electron. 24, 2017 (1988), or J. Kinoshita et al. IEEE J. Q. Electron. 27, 1759 (1991)]. Not all the advantages which are offered by an arbitrary continuous distribution of the phase shift were used.
Furthermore, the sinusoidal variation of refractive index by means of stripe width varied in the axial direction is known. This solution was suggested theoretically to achieve a complete single-mode emission [K. Tada et al. Electron. Lett. 20, 82 (1984)].
Furthermore, a continuous variation of refractive index in the axial direction has been described theoretically [J. Lightwave Technol. 11, 1325 (1993)]. The axial variation of the refractive index was indicated by means of lateral symmetrical reduction of the grating field width in an optical waveguide.
Furthermore, a solution for producing phase shifts is known, which is based on an expansion of the vertical thickness of the active layer or of adjacent waveguide layers over a specific length of the longitudinal component length [e.g. B. Broberg, et al. Appl. Phys. Lett. 47, 4 (1985) or K. Kojima et al. J. Lightwave Technol. LT-3, 1048 (1985)]. This leads to undesired additional optical multiple reflections and optical interference effects at the points at which the refractive index changes abruptly. In addition, the advantages which are made possible by a continuous phase shift distributed arbitrarily over a longer partial section, were not exhausted by a long way.
Further known solutions contain the generation of a phase shift via the axial variation of the grating period.
In the literature, examples are quoted for the abrupt changes of the grating period in the axial component direction. In the centrally disposed section of the laser resonator, a larger grating period was implemented holographically than in the side sections. This structure could be successfully used for reducing the optical line width [M. Okai et al., IEEE J. Quantum electron. 27, 1767 (1991)]. The central region of changed grating period serves for generating the phase shift. Abrupt grating period changes were generated with this structure. However, not all the advantages which are offered by a continuous variation of the coupling coefficient, were exhausted.
Within a specific framework, electron beam lithography (EL) also enables the implementation of phase shifts distributed in the local space via the changing of the grating period in the longitudinal direction. However, in the case of this process, the difference between adjacent grating periods is limited to larger values. As a consequence, only DFB gratings which have a small number of different sections, within which the grating period is constant, but which differ from section to section, can be produced using EL. No quasi-continuous variations of the grating period with the location can be achieved. Furthermore, EL is a complicated process and the EL pattern-writing time is very expensive.
Curved waveguides on homogeneous DFB or DBR grating fields can, as already known, be used for the definition of gratings with axially varying grating period. Using this method, defined phase shifts can also be generated via a deliberate axial variation of the grating period and the phase shifts simultaneously distributed axially in an arbitrary and quasi-continuous manner.